Bacterial chemotaxis has emerged as one of the very best characterized examples of information processing in a biological system. Extra-cellular information is converted into a usable intracellular form via a signal transduction network. This system is not only well characterized and experimentally accessible but it also exhibits some important characteristics of biological complexity. The chemotactic system in E. coli possesses all the key properties that allow living cells to transduce and to respond to environmental signals. [We will study the molecular origin of behavioral variability in individual cells in order to understand the underlying molecular mechanisms of the chemotaxis signal transduction network. In particular we will analyze the significance of the observed variability in single cells in the chemotaxis response at the population level.] We shall put a special emphasis upon the quantitative aspects of these studies where experimental data will be compared with the results of numerical simulations.